drm/vkms: Remove event from vkms_output

While working on rvkms, I noticed that there's no code that actually uses
the drm_pending_vblank_event that's embedded in vkms_output. So, just drop
the member from the struct.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Tested-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240703160458.1303872-1-lyude@redhat.com